1. Field of the Invention
The present invention relates to a wireless communication apparatus and a method for compensating offset using the same, and more particularly, to a wireless communication apparatus and a method for compensating offset using the same in which latency generated in a power save mode can efficiently be compensated.
2. Description of the Related Art
Bluetooth is a name for a wireless data communication technology in the field of electrical communication, networking, computing, and consumer goods. The Bluetooth technology serves to connect various apparatuses through one wireless connection operating over a short distance. For example, if the Bluetooth wireless communication technology is implemented in cellular phones and laptop computers, the cellular phones and laptop computers can communicate among themselves without cables. All digital equipment as well as printers, personal digital assistants (PDA), desk top computers, facsimiles, keyboards, and joy sticks may be a part of the Bluetooth system.
Generally, the Bluetooth system has a maximum data transmission of 1 Mbps and a maximum transmission distance of 10 m. 1 Mbps is a frequency within an industrial scientific medical (ISM) frequency band of 2.4 GHz that can be used without any permission, and can readily be realized at a low cost. Also, the transmission distance of 10 m has been established because it is a sufficient transmission distance between a user's portable equipment and a personal computer on a desk in an office.
Furthermore, since the Bluetooth system has been devised to operate in the radio frequency environment where a lot of noise exists, a frequency hopping system of 1600 times per second is used to provide stable data transmission and reception in such a radio frequency environment having a lot of noise.
Moreover, the Bluetooth system supports one-to-multiple connection as well as one-to-one connection. As shown in FIG. 1, in the Bluetooth system, a plurality of piconets can be constructed and connected, respective piconets being divided by different frequency hopping orders. The term “piconet” means a Bluetooth unit formed by connecting one or more slaves to one master device. A piconet can have one master and a maximum of seven slaves. The master equipment and the slave equipment implement bi-directional communication by means of time division duplex (TDD) based on 1 hopping slot of 625 μs (1/1600 second). The plurality of piconets systematically connected with one another constitute a scatternet.
FIG. 2 illustrates communication between the master and the slaves by means of the TDD. Referring to FIG. 2, each channel allocated to time slots has a length of 625 μs. The number of time slots is determined by a Bluetooth clock of the piconet master. Also, the master and the slave can alternatively transmit packets. That is, the master transmits packets in only the time slot marked by an even number. The slave transmits packets in only the time slot marked by an odd number. The packets transmitted by the master or the slave should be provided within five time slots. A packet is a unit of data transmitted on a piconet channel.
The piconet is synchronized with the system clock of the master. The system clock of the master is never corrected as long as the piconet exists. The system clock of the master is maintained in a period of M×625 μs during successive transmissions, where M is an even number and a positive integer.
The slave updates timing offset to match with the master clock. That is to say, the slave compares a test RX timing with an exact RX timing of a received packet so as to update offset for compensating a timing error.
The slave TX timing should be based on the latest slave RX timing. The RX timing is based on the last successive trigger during a slot from the master to the slave. The slave can receive a packet and can correct the RX timing as long as timing inconsistency remains in a window of uncertainty of ±10 μs.
When the Bluetooth system is connected, the master can manage the slave in a hold mode, a sniff mode, a park mode, and so on, in order to save power. The hold mode retains an active member address AM_ADDR in a state where the master is connected with the slave, and proceeds into a sleep state. The sniff mode retains an active member address AM_ADDR in a state where the master is connected with the slave, and extends a listen interval. The park mode proceeds into a sleep state by opening an active member address AM_ADDR in a state where the master is connected with the slave. Before the slave proceeds into the park mode, a park mode address PM_ADDR or an access request address AR_ADDR is assigned from the master.
FIG. 3 illustrates an RX/TX cycle of a Bluetooth master in a normal mode for a single slot packet, and FIG. 4 illustrates an RX/TX cycle of a Bluetooth slave in a normal mode for a single slot packet. As shown in FIGS. 3 and 4, the Bluetooth system selectively transmits and receives a packet in a state where the master is connected with the slave. The maximum size of the packet may be 366 μs depending on the type of packet and the load length. The RX transmission and the TX transmission have different hopping frequencies. The channel hopping frequency is denoted by g(m).
Once the transmission is implemented, the return packet is estimated as N×625 μs after a TX burst starts, wherein N is an odd number and a positive integer. Also, N depends on the type of the transmitted packet. The window has a length of 20 μs during normal operation, which assures latency of +/−10 μs.
If the master of the Bluetooth system is transited to an active state in a state where it manages the slave in a hold mode, a sniff mode, or a park mode, offset may occur between the master and the slave. That is, latency of +/−10 μs is assured while the Bluetooth system is connected. If such latency is accumulated in a power save mode for a long time (>40 sec.), the Bluetooth system, which employs a frequency hopping spread spectrum (FHSS), fails to transmit data even under the connection state. Referring to FIG. 5, the Bluetooth system may be in a hold mode under the connection state. In the hold mode, the Bluetooth system neither transmits nor receives data. If the slave of the Bluetooth system is returned to a normal mode, the slave should check, before transmitting data to the master, whether there has been any request signal from the master. In this case, the size of a search window of the slave can increase from +/−10 μs to Xμs as shown.
If the size of the search window exceeds 625 μs, the following window starts not at the RX hopping frequency g(2m), g(2m+2), . . . , g(2m+2i) (where, i is an integer) but at g(2m), g(2m+4), . . . , g(2m+4i). Alternatively, the following window starts at g(2m), g(2m+6), . . . , g(2m+6i). In this case, a problem arises in that inconsistency of the hopping frequency is caused, thereby exceeding a limited time of 40 sec.
To prevent such a problem from occurring, an experimental estimated value, derived from many experiments, should be provided. In this case, however, power conservation features may be lost. Also, if a hold mode of 40.9 sec. is used, the maximum latency of 32.750 ms should be compensated, thereby exceeding 625 μs.